Personally I wonder why a nation with access to helicopters has such difficulty in a war with people mostly armed with knives.
Because NCR's economy took a hit during the BoS war. Vertibirds must cost big bucks and that kinda money could go to building things like rail roads and roads, towns, power plants, food production, education and heathcare. NCR is huge and their resouces are going to all over. The Mojave campaign was to be a small campaign. If the president has to send in Vertibirds and a lot more troops it will make him look bad. The legion is also huge so NCR can't deal with an all out war against such a large power a power they did not expect to run into.